hepatic vasculature described with doppler ultrasound

1. Hepatic Doppler US [2]
Dr. Kamal Sayed MBBS MSc US UAA

2. Color AND spectral parameters [contd..]
[3] Wall Filters
•
Wall filters @ selectively filter out all acquired information
below an operator-defined frequency threshold.
•
@ Filters eliminate the typically low-frequency–high-intensity
noise that may arise from vessel wall motion.
@ Filters operate at variable frequencies to eliminate signal from
low-velocity blood flow.
@ Filter settings are usually preset by the manufacturer, and a
high, medium, or low filter setting may be applied separately
to spectral, color, and power imaging.

3. •
“High” refers to the higher range of frequency shifts that are
filtered out and thus are not depicted on the color image or
spectral waveform.
•
In patients with very slow portal venous flow, a high filter
setting may cause this flow to be inadvertently
[unintentionally] obscured or missed.
•
To avoid the loss of signal that characterizes slow flow, filter
settings should be kept at the lowest possible setting
(typically in the 50–100-Hz range).
•
.

4. •
The cutoff frequencies vary with the velocity scale; the
lowest filter cutoff frequencies cannot be used with the
highest velocity ranges and vice versa.
•
On the color bar, the filter setting is indicated by black areas
on both sides of the baseline.
•
•

5. # Expansion of the filter shows up as a widening of the black
band (,,,Fig 10) slide [7]
# On the spectral waveform, a high wall filter setting will
•
result in loss of depicted spectral information immediately
above the baseline. Reducing the wall filter setting results in
filling in of spectral data toward the baseline.
•

6. •
Figure 9. Changing the wall filter.
•
(a) Color duplex US image [slide [7] obtained with a high wall
filter setting shows @ loss of the low-velocity-flow
component of the spectral waveform immediately above the
baseline. @ Higher-velocity flow is well depicted, and
@accurate flow quantification can still occur.
•
(b) Color duplex US image slide [8] demonstrates how the
spectral waveform progressively fills in toward the baseline
as the wall filter is sequentially reduced from high (left arrow)
to medium (middle arrow) to low (right arrow).

7. Figure 9 [a] Changing the spectral wall filter [high filter]

8. Figure 9 [b] Changing the spectral wall filter [reduced filter from high to med
to low = arrows]

9. •
Figure 10. Changing the color Doppler wall filter.
•
(a) CFD image slide [10] obtained with the highest possible
wall filter setting shows how color signal arising from low-
velocity flow may be filtered out.
•
## The change in the filter setting appears as a change in the
width of the horizontal black line in the center of the color
bar. Slide [10]
•
(b) CFD image slide [11] obtained with a low filter setting
demonstrates filling in of flow in the hepatic veins (blue),
which indicates minimal filtering of color signal.
•

10. Figure 10 [b] (high filter = filters out signals from low flow velocities)
The change in the filter setting appears as a change in the width of the
horizontal black line in the center of the color bar.

11. Figure 10 [b] filling in of flow in hepatic veins [blue] Changing the color
Doppler wall filter from high to low.
•

12. •
[4] Inversion of Flow figure 11
•
Inversion refers to the ability to electronically invert the
direction of flow as depicted on both the color flow and
spectral waveforms.
•
a) As such, color inversion will result in a blue-red reversal
and may lead to misinterpretation of the direction of flow in
the vessel being evaluated (Fig 11 a/slide 13).
•
b) On a color Doppler flow US image obtained with reversal of
this inversion, appropriate directional flow is noted.
•
[Figure 11 b/slide 14]

13. Figure 11a. Inversion of color flow. Image [color Doppler active mode] on
inversion of the color bar, portal venous flow appears blue, which falsely
suggests reversal of flow (ie, away from the transducer).

14. Figure 11b. Inversion of color flow. On a color Doppler flow US image
obtained with reversal of this inversion, appropriate directional flow is
noted.

15. •
Similarly, inversion of the spectral waveform will switch the
depicted flow curve from above to below (or below to above)
the baseline (,,,Fig 12).
•
Adjustment of the inversion button will alter either the color
or the spectral scale, depending on which is active at the time
of adjustment.
•
Flow toward the TXR typically appears red (at CFD) OR above
the baseline (positive flow) on the spectral waveform and can
be changed simply by adjusting the inversion button.

16. •
Figure 12. Inversion of spectral and color flow falsely suggesting
reversal of portal venous flow.
•
(a) On a color duplex image slide [17] obtained with spectral
Doppler as the active mode, @ the spectral waveform is
below the baseline, @ with appropriate color flow.
•
(b) Color duplex image slide [18] obtained after the inversion
button was reversed demonstrates @ appropriate directional
flow, with the spectral @ waveform now appearing above the
baseline.
•
Note that the color bar does not change when the Doppler
spectrum is inverted.

17. Figure 12 a. Inversion of spectral AND color flow : spectral [active mode], the spectral waveform
is below the baseline, with appropriate color flow.

18. Figure 12 b. Color duplex obtained after the inversion button was reversed
demonstrates appropriate directional flow, with the spectral waveform now appearing
above the baseline.

19. •
(B) Spectral-Specific Parameters
•
[angle correction/ spectral gain/ gate size/ gate position]
•
The spectral waveform contains a host of hemodynamic
information describing the velocity and character of the
blood flow in the specific vessel being insonated.
•
The waveform depicts the spectrum of frequency shifts of all
blood traversing the sampled volume during the period of
data acquisition . With use of the computer to apply an angle
correction, a range of time-dependent velocities are
displayed on the vertical axis (spectral display).

20. •
[1] angle correction
•
Angle correction refers to adjustment of the Doppler angle
and is used to calibrate the velocity scale for the angle
between the US beam and the blood flow being measured.
•
Spectral Doppler US provides both qualitative and
quantitative data about flow velocity.
•
Flow velocity is calculated from the Doppler frequency shift
according to the Doppler equation :
•
V = ΔfC/2focosθ

21. •
Flow velocity is calculated from the Doppler frequency shift
according to the Doppler equation :
•
V = ΔfC/2focosθ
•
where
•
V = velocity.
•
Δf = Doppler frequency shift.
•
C = speed of sound in soft tissue.
•
fo = original transmitted frequency.
•
θ = the angle between the transducer and blood flow.

22. •
Ideally, the direction of flow should be at an approximately
45°–60° angle relative to the transducer.
•
Within this range, a linear relation exists b/w velocity and the
Doppler shifts.
•
Outside this range, the unreliable signal produces inaccurate
estimates of flow (,,,Figs 13, ,,,14).
•
The calculated velocity is inversely related to : the cosine of
the angle b/w the TXR and flowing blood.
•
Because the cosine of 90° is 0, the calculated velocity
approaches 0 as the angle approaches 90°

23. •
Figure 13. Angle correction.
•
(a) Color duplex US image [slide 24] obtained with no angle
correction [angle 0^] shows how no meaningful velocity data
can be obtained from the portal venous waveform because
the computer automatically assigns an angle of 0° (cos 0° = 1).
•
(b) Color duplex US image slide [25] obtained with correct
definition of the angle between the TXR and the direction of
portal venous flow [angle 52^] demonstrates a flow velocity
of 29.3 cm/sec.

24. Figure 13 a : Without angle correction [anle 0^], the measured flow velocity is 18.0
cm/sec.
no meaningful velocity data can be obtained [computer automatically assigned
anangle of 0^ [cosine 0 = 1]

25. Figure 13 b : after angle correction [52^], the measured flow velocity is
29.3 cm/sec.

26. •
Figure 14. Angle correction.
•
(a) Color duplex image slide [27] obtained with a 30°
corrected angle, which is too low, demonstrates a flow
velocity of 21.3 cm/sec [underestimation of true portal vein
flow velocity].
•
(b) Color duplex image slide [28] obtained with a 70°
corrected angle, which is too high, demonstrates a flow
velocity of 52.8 cm/sec in the portal vein, which represents an
overestimation of flow velocity.
•
Note that the measured flow velocity increases as the
corrected angle increases.

27. Fig 14 a : with 30^ corrected angle [too low] = duplex image demonsrated
overestimated flow velocity of 21.3 cm/s

28. Fig 14 b : with 70^ corrected angle [toohigh] = duplex image demonsrated
low velocity of 52.8 cm/s

29. •
The differences between angle correction and angle of
insonation are important to understand.
•
Angle correction specifies the true Doppler angle [b/w beam
& blood flow] by placing the cursor parallel to the direction of
blood flow (,,,Fig 13).
•
Manually applying this correction allows the computer to
solve the Doppler equation.
•
The angle of insonation is the angle b/w theTXR and the
vessel being studied (,,,,Fig 15). The angle of insonation
should also be between 45° and 60°.

30. •
Figure 15. Angle of insonation.
•
(a, b) Color duplex image [slide 31] of the anterior branch of
the right portal vein obtained with the TXR positioned in an
intercostal (a) and subcostal (b) location depict flow as
moving toward [a] and away [b] from the TXR, respectively.
•
(c) On a color duplex image obtained with theTXR positioned
perpendicular to flow (arrow), no color is assigned, yielding a
false finding of absent flow.
•
The angle of insonation of the vein depends entirely on the
position of the transducer.

31. Figure 15 a :

32. Figure 15 b :

33. Figure 15 c :

34. •
UNdercorrection applied to the Doppler angle, will result in a
falsely low flow estimate.
•
This result comes directly from the Doppler equation, in
which the smaller the angle between flow direction and
transducer position, the greater the denominator of the
equation.
•
Flow may appear to be reversed when the beam-flow angle
changes about 90°.
•
Complete loss of flow may be evident when the beam-flow
angle is 90°.

35. •
[2] Spectral Gain
•
The spectral gain setting enhances the intensity of depicted
flow in the spectral display.
•
Gain should be adjusted to outline the contour of the
depicted waveform (,,,,,Fig 16/).
•
Too low a setting falsely suggests absent flow.
•
Too high a setting artificially fills in the spectral waveform,
resulting in falsely increased flow with little meaningful
quantitative flow data.

36. •
gain settings function independently of other parameters,
•
so that changing the percentage of gain applied to an
acquired signal will not alter any other parameter.
•
A change in the color gain does not alter the spectral gain and
vice versa, with the PRF remaining unchanged.
•
Figure 16. Optimization of gain settings.
•
(a) Duplex image [slide [38] obtained with spectral Doppler as
the active mode and too low a gain setting (0%) falsely
suggests absent flow.

37. •
(b-d) Duplex images slides [39/40/41] obtained with a gain
setting of 38% (b), 77% (c), and 100% (d) demonstrate gradual
artificial filling in of the spectral waveform, yielding a false
finding of increased flow with little meaningful quantitative
flow data. changing the gain setting will not alter any other
parameter.
•
Changing the color gain does not alter the spectral gain (and
vice versa), and the PRF remains unchanged. Whether the
color or spectral component is active, the gain setting should
be adjusted to outline the contour of the depicted waveform
or color flow depiction.
•

38. Figure 16 a : spectral Doppler, active mode, too low a gain setting (0%) =
falsely suggests absent flow

39. Figure 16 b : gain setting 38%

40. Figure 16 c : gain setting 77%

41. Figure 16 d : gain setting 100%

42. •
[3] Gate Size
•
The operator-adjusted gate defines the size and location of
the area from which Doppler information is obtained. The
gate is delineated as a pair of cross-hairs within the 2D image
and should be as small as possible to exclude erroneous signal
arising from adjacent vessels or marginal flow.
•
Too large a gate may @ admit erroneous signal from adjacent
vessels (,Fig 17) @ or may lead to acquisition of data from
extraneous parenchyma.

43. •
Too small a gate may @ give the false impression of reduced
•
or even absent flow. @ A smaller gate also reduces
computation time @ and increases the frame rate, thereby
@allowing more accurate depiction of flow.

44. •
Figure 17. Optimizing gate size and position.
•
Color duplex US image slide [45] obtained with @ a wide gate
placed in @ a suboptimal location : shows sampling of flow in
both the portal (above the baseline) and hepatic (below the
baseline) veins.
•
Too large a gate size may result in sampling from too large an
anatomic region.
•
By reducing the gate size and improving the position for
sampling, a normal spectral waveform is obtained.

45. Figure 17. Optimizing gate size and position

46. •
[4] Gate Position
•
To maximize depiction of flow, the gate should be positioned
over the central part of the vessel being studied. In central
portions of the liver, where portal and hepatic veins course in
proximity to one another, a small gate should be carefully
placed on the desired vessel to avoid obtaining flow patterns
from adjacent vessels (,Fig 17/slide 45). When helical flow
occurs in the portal vein, as is often seen following liver
transplantation, gate placement over the central part of the
vein will demonstrate expected flow above and below the
spectral baseline.